"Don't know" is the answer from me (hopefully someone else does).

But this is pretty fundamental, I would have thought, and there mightn't be
any information about it on the net.

First thing, though, is to check your references. Maybe something has got
itself turned off. Looking at an empty spreadsheet that compiles your
statement without trouble, the references that are ticked (Excel 2003) are:

Visual Basic For Applications
(In Program Files\Common Files\Microsoft Shared\VBA\VBA6\VB........ can't
read the rest)

Microsoft Excel 11.0 Object Library
(In Program Files\Microsoft Office\OFFICE11\EXCEL.EXE)

OLE Automation
(In WINDOWS\System32\stdole2.tlb)

Microsoft Office 11.0 Object Library
(In Program Files\Common Files\Microsoft Shared\OFFICE11\M........ can't
read the rest)

Microsoft Forms 2.0 Object Library
(In WINDOWS\System32\FM20.tlb)

I don't know which of these are default and which I've turned on at some
stage, but it'd be worth checking that they're on in yours.